Magnetic fluid seal with centering of bearing and shaft by compressible member

ABSTRACT

The present disclosure describes a magnetic fluid sealing device having a shaft centered with respect to a magnetic structure and rolling element bearing, as well as a method for centering the shaft in the device. A compressible ring located in a groove on the shaft is used to partially fill the gap between the shaft and the rolling element bearing and to make contact with the rolling element bearing. The compressible ring aligns and centers the shaft with the rolling element bearing. A liquid locking material is added to the gap and hardened to couple the shaft and compressible ring to the rolling element bearing. An alternative self-alignment mechanism is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/104,181 filed on Oct. 9, 2008, entitled “MAGNETIC FLUID SEAL WITHCENTERING OF BEARING AND SHAFT BY COMPRESSIBLE MEMBER,” the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

This disclosure relates generally to magnetic fluid seals, and morespecifically to a method for centering the seal about a rotatable shaft,sleeve, or the like.

BACKGROUND OF THE INVENTION

Magnetic fluid seals are commonly utilized to provide a seal that willprotect devices against the introduction of gas or other contaminants.These seals may be installed to provide a barrier between variouscomponents present in a device that are either stationary with respectto each other or in a rotational relationship. For example, magneticfluid seals have been utilized in computer magnetic disc storage unitsas a barrier against contaminants being transmitted between the motorarea and the disc area. Magnetic fluid seals also have been designed toseal robotic actuators and to seal around rotatable x-ray tubes that areused in high vacuum environments, as well as to seal rotary componentsincorporated into pumps used in refineries and chemical plants.

Magnetic fluid seals generally operate through the placement of amagnetic fluid (e.g., a ferrofluid) in the gap established between thesurface of a rotating shaft and stationary surface for use as a dynamicseal. The stationary surface normally includes an annular magneticstructure whose peripheral edge forms a close, non-contacting gap withthe surface of the rotating shaft. The magnetic flux path generated bythe magnet retains and concentrates the magnetic fluid in the gapforming a tight seal that resembles a liquid o-ring. Since the rotatingand stationary surfaces do not directly contact each other, they aresubject to very little wear. Thus the serviceable life of the magneticfluid seal is remarkably extended in comparison to the life expectancyof a mechanical seal.

In order for a magnetic fluid seal to operate properly, it is importantthat the annular magnetic structure, including any magnets andcorresponding pole pieces, is mounted concentrically about the rotatableshaft. Inaccurate centering about the shaft will result in a non-uniformwidth in the annular gap established between the magnetic structure andthe shaft. Generally, it is the pole pieces which are in closestproximity with a shaft and for which centering is most critical.

When a magnetic seal is mounted non-concentrically about the shaft, theresulting magnetic field will not be symmetrically distributed aboutthis shaft, but rather the magnetic flux will be elevated near thenarrowest portion of the gap and reduced near the gap's widest portion.An uneven distribution of magnetic flux within the annular gap may causethe magnetic fluid to concentrate towards the narrowest portion of thegap, thereby, leaving the widest portion of the gap with an insufficientvolume of fluid to maintain the desired sealing strength. A reduction insealing strength can lead to seal “bursting” at a lower thresholdpressure differential across the seal than one skilled-in-the-art wouldnormally expect or predict. Accordingly, there exists a continual desireand need to provide magnetic fluid sealing devices, and a method ofcentering the magnetic seals about rotatable shafts.

SUMMARY OF THE INVENTION

The present disclosure provides a magnetic fluid sealing device having ashaft centered with respect to a magnetic structure and to a rollingelement bearing, as well as a method for centering the shaft in thedevice. One embodiment of a magnetic fluid sealing device, constructedin accordance with the teachings of the present disclosure, generallycomprises a housing; a shaft having a first portion and a second portionalong its longitudinal axis (x); a magnetic structure having acylindrical channel sized to encircle the first portion of the shaft,thereby, forming a first radial gap; a magnetic fluid located within thefirst gap; a compressible ring; and a rolling element bearing having aninner ring and an outer ring, the inner ring being sized to encircle thesecond portion of the shaft, thereby, forming a second radial gap. Theouter surface of the magnetic structure and the outer ring of therolling element bearing are coupled to the housing such that the centerof the inner ring of the rolling element bearing and the center of thecylindrical channel in the magnetic structure are coaxially aligned.

The outer surface of the shaft in its second portion includes at leastone groove radially encircling the shaft in which a compressible ring islocated or seated. The compressible ring is further adapted to fill apart of the second gap and to contact the inner ring of the rollingelement bearing. The contact between the compressible ring and the innerring of the rolling element bearing radially aligns and centers theshaft along its longitudinal axis (x) with the bearing, while themagnetic fluid establishes a seal between the magnetic structure and theshaft.

According to another aspect of the present disclosure, a hardenablelocking material is placed into the second gap. This locking materialupon hardening couples the shaft and the compressible ring to the innerring of the rolling element bearing. Optionally, the shaft/bearingassembly may be placed into an external fixture to maintain alignmentbetween the shaft and the bearing while the locking material hardens.

According to yet another aspect of the present disclosure, the shaft mayinclude a shoulder upon which the groove and compressible ring reside.The surface of this shoulder is square or normal to the longitudinalaxis of the shaft. This shoulder in conjunction with the compressiblering can center and align the shaft and the roller bearing, as well asmaintain such alignment while the locking material hardens without theuse of an external fixture.

Another objective of the present disclosure is to provide a method ofassembling the magnetic fluid sealing device described herein. Thismethod generally provides a shaft with at least one groove in which acompressible ring is fit or seated, a magnetic structure, and a rollingelement ring in which the center of the concentric channels of each arecoaxially aligned when coupled with the housing. The insertion of theshaft into the magnetic structure establishes a first radial gap, whilethe insertion of the shaft into the inner ring of the rolling elementbearing establishes a second radial gap. The insertion of the shaftthrough the inner ring of the rolling element bearing is done such thatthe compressible ring fills part of the second gap and makes contactwith the inner ring. This contact with the inner ring allows thecompressible ring to radially align and center the shaft along itslongitudinal axis (x) with the rolling element bearing. A magnetic fluidis placed into the first gap in order to establish a seal between themagnetic structure and the shaft. A hardenable locking material is addedto the second gap. When this locking material hardens, it couples theshaft and compressible ring to the inner ring of the rolling elementbearing.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a cross-sectional schematic of a magnetic fluid sealing deviceconstructed according to the teachings of the present disclosure;

FIG. 2A is a magnified view of section 2(ABC) of the sealing device ofFIG. 1 highlighting the second gap formed between the rolling elementbearing and the shaft according to one embodiment of the presentdisclosure;

FIG. 2B is a magnified view of section 2(ABC) of the sealing device ofFIG. 1 highlighting another aspect of the present disclosure;

FIG. 2C is a magnified view of section 2(ABC) of the sealing device ofFIG. 1 highlighting yet another aspect of the present disclosure;

FIG. 3 is a schematic representation of a method for assembling themagnetic fluid sealing device of FIGS. 1, 2A, and 2B according to theteachings of the present disclosure; and

FIG. 4 is a schematic representation of a method for assembling themagnetic fluid sealing device of FIG. 2C according to another aspect ofthe present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The following description is merely exemplary in nature and is in no wayintended to limit the present disclosure or its application or uses. Itshould be understood that throughout the description and drawings,corresponding reference numerals indicate like or corresponding partsand features.

The present disclosure generally provides a magnetic fluid sealingdevice for incorporation into an apparatus having rotatable components.More specifically, the magnetic fluid sealing device uses a rollingelement bearing that is aligned and centered with a rotatable shaftthrough the use of a compressible ring and/or shoulder located on thesurface of the shaft. The use of such a compressible ring or shoulder onthe shaft allows one to quickly establish a small, circumferentiallyuniform gap between an inner surface of a magnetic structure in themagnetic sealing device and the outer surface of a rotatable shaft. Themagnetic sealing device of the present disclosure overcomes the variousdrawbacks and problems associated with conventional approaches tocentering a rotatable shaft in a magnetic sealing device.

Referring to FIG. 1, the magnetic sealing device 10 comprises a housing15 mounted to an apparatus in which a magnetic structure 35 and arolling element bearing 50 are coupled. A shaft 20 is also rotatablymounted in this housing 15. The shaft 20 is defined as having an outersurface 22 divided into a first portion 24 and a second portion 26 alongits longitudinal axis (x), such that the delineation between the firstportion 24 and second portion 26 resides between the magnetic seal androlling element bearing 50. The outer surface 22 of the shaft 20 in itssecond portion 26 has at least one groove 30 that encircles the shaft20. A compressible ring 70 whose cross-sectional diameter is greaterthan the depth of the groove 30 is located or seated in this groove 30.The magnetic fluid sealing device 10 as shown in FIG. 1 is symmetricalaround the shaft 20.

The housing 15 may be mounted to an apparatus using any known meansknown to one skilled-in-the-art, such as a flange and bolt combination,among others. The housing 15 is normally constructed out a metal, thecomposition of which can vary depending upon the application and desiredperformance characteristics. However, the housing 15 may be constructedout of any material, including metal or composites, as desired.

The magnetic structure 35 is defined by an inner surface 37 and an outersurface 38. The outer surface 38 of the structure 35 is coupled to thehousing 15. The inner surface 37 of the structure 35 is sized to have adiameter that encircles the outer surface 22 of the shaft 20 along itsfirst portion 24, thereby, creating a first radial gap 40. A magneticfluid 45 is located in this first gap 40 to establish a seal between themagnetic structure 35 and the shaft 20.

The rolling element bearing 50 is defined by an inner ring 55 and anouter ring 60. The outer ring 60 represents the part that is coupled tothe housing 15. Retainers 51 may be used to secure the inner ring 55 andouter ring 60 of the rolling element 50 bearing in place. The inner ring55 has an inner surface 62 that is sized to have a diameter thatencircles the outer surface 22 of the shaft 20 along its second portion26, thereby creating a second radial gap 65.

Referring now to FIGS. 1 and 2A, the compressible ring 70, which islocated in the groove 30 on the shaft 20, is adapted to fill a portionof the second gap 65 and contact the inner ring 55 of the rollingelement bearing 50. It is this compressible ring 70 that radially alignsand centers the shaft 20 along its longitudinal axis (x) with therolling element bearing 50.

One skilled-in-the-art will understand that the first portion 24 of theshaft 20 is part of the magnetic circuit created during the functioningof the magnetic fluid seal and therefore is preferably constructed of amagnetically permeable material. The first portion 24 of the shaft 20may incorporate magnets 42 and pole pieces 43 as shown in FIG. 1. Inthis case, the magnets 42 and pole pieces 43 will rotate with the shaft20 and define the outer surface 22 of its first portion 24. A pole piece43 represents the portion of a magnetically permeable material throughwhich the lines of magnetic flux become concentrated. In the magneticfluid device of the present disclosure, the pole pieces are typicallylocated near the surface of the shaft 20 and magnetic structure 35 inorder to effectively attract and retain any magnetic fluid that ispresent in the first radial gap 40.

Magnetic stainless steel is the preferred material for the outer surface22 of shaft 20 because it provides a desirable combination ofproperties, such as cleanliness, vacuum compatibility, ferromagneticpermeability, and relatively poor thermal conductivity. The preferredmagnetic stainless steel is 17-4PH alloy, also known as alloy 630.Stainless steel alloys of the so-called 400 series can also be used, andmay be desired in some applications, such as applications where a veryhigh hardness is desirable. Magnetic alloys other than stainless steelmay be used, but are not normally desired for use in many applicationsbecause of their incompatibility with a high vacuum environment or otherprocess environments (e.g., corrosive gases).

The shaft 20 may be solid or hollow depending upon the function of theapparatus in which the magnetic fluid sealing device is utilized. Inother words, the core of the shaft 20 may when desired further define apassageway, channel, duct, or conduit. Thus the shaft 20, which istypically cylindrical in shape, may be one selected from the group of arod, an axle, a tube, a sleeve, and a pipe. The shaft 20 may berotationally driven by a motor or other means (not shown).

As shown in FIG. 1, the magnetic structure 35 may be comprised of amagnetically permeable material, such as magnetic stainless steel. Theinner surface 37 of the magnetic structure acts as a pole piece 42 withrespect to concentrating the magnetic flux to attract and retain themagnetic fluid 45 present in the first radial gap 40. In this case, theouter surface 22 of the rotatable shaft 20 in its first portion 24 willneed to be comprised of both magnets 42 and pole pieces 43 in order tocomplete a magnetic circuit. The magnetic structure 35 is preferablyarranged, such that its inner surface 37 forms a small clearance orfirst gap 40 with the outer surface 22 of the shaft 20. Thus themagnetic structure 35 forms a cylindrical cavity whose diameter isslightly larger than the outside diameter of shaft 20. This type ofmagnetic circuit is generally called a “SuperSeal” configuration.

Alternatively, the magnetic structure 35 may be comprised of acombination of annular magnets 42 and pole pieces 43 of a magneticallypermeable material, such as magnetic stainless steel. In this case, theouter surface 22 of the shaft 20 in the first portion 24 will be a polepiece 43 or combination of pole pieces 43 made from a magneticallypermeable material. The magnets 42 in this alternative arrangementremain stationary as the shaft 20 rotates. This type of magnetic circuitis generally called a “Stationary Pole Piece” configuration. Thus oneskilled-in-the-art will understand that the magnetic sealing device 10of the present disclosure may include arrangements where the magnets 42and pole pieces 43 are either part of the magnetic structure 35 andremain stationary upon the rotation of the shaft 20 or are part of theshaft's 20 first portion 24 and rotate with the shaft 20.

The distance between the magnetic structure 35 and shaft 20, whichestablishes the first radial gap 40, may be on the order of about 0.001to 0.004 inches. The magnets 42 are arranged so that the polarity of themagnets 42 on the opposite side of the pole piece 43 is symmetrical withrespect to the pole piece 43. Thus, the polarity of the oppositesurfaces of the two adjoining magnets 42 is the same as each other.

Rare earth magnets 42, such as SmCo or NdBFe, are preferred. Thesemagnets 42 can be used as a single component or arranged in layers. Anynumber of magnet layers can be used, but an even number is preferred(for cancellation of fringe fields). One layer is sufficient for allvacuum applications, although two layers are normally desirable. Forapplications with large pressure differentials, a greater number oflayers can be used. The surface of the pole pieces 43 may be continuousalong the longitudinal axis of the magnetic seal. Such a continuoussurface may include the presence of one more grooves, commonly referredto as pole tips.

A magnetic circuit is established by the combination of the magneticstructure 35, the first portion 24 of the shaft 20, and the magneticfluid 45. The magnetic field created by the magnet 42 follows a fluxpath through the pole pieces 43 in the shaft 20 and magnetic structure35. The flux path extends across the first radial gap 40 in which themagnetic fluid 45 is located. The magnetic flux retains the magneticfluid 45 in the gap 40, thereby forming a liquid o-ring seal aroundshaft 20.

One skilled-in-the-art will understand that the magnetic fluid 45 may beany ferrofluid composition known to function as a magnetic seal. Suchmagnetic fluids 45 generally comprise a carrier fluid such as water, ahydrocarbon, or a fluorocarbon; ferromagnetic particles, such as ironoxide or ferrite dispersed in the carrier fluid; and a surfactant, suchas a fatty acid, to assist in the dispersion of the particles. Themagnetic fluid 45 is placed in the first radial gap 40.

The cylindrical cavity formed by the magnetic structure 35 is coaxiallyaligned with the cavity formed by the inner ring 55 of the rollingelement bearing 50. Since the alignment of the shaft 20 is accomplishedwith the use of the rolling element bearing 50, when the magneticstructure 35 is positioned within the housing 15 and affixed thereto,the magnetic structure 35 is, ideally, automatically centered about theshaft 20. The rolling element bearing 50 has a predetermined amount ofstiffness that provides a resistance large enough to offset theoccurrence of any radial loading or force moments which could tilt theshaft 20 once the shaft 20 and roller element bearing 50 are coupledaccording to the teachings of the present disclosure.

Rolling element bearings 50 with large radial and axial play (e.g.,radial ball bearings) should be avoided in favor of preloaded bearings.In order to achieve high stiffness in conventional preloaded bearingsets, it is usually necessary to spread the bearings apart axially bymeans of matched-length spacers. However, the use of such spacersgenerally increases the overall length of the feed through and resultsin the bearing occupying a greater amount of space in the overallmagnetic fluid sealing device. The bearing preferred in this embodimentis a rolling element bearing 50 of the type manufactured by THKCorporation, Tokyo, Japan, which offers very high stiffness in anextremely short axial space.

The magnetic fluid seal established through the magnetic fluid 45 in thefirst gap 40 may be used to separate a low pressure region in anapparatus from a region in the apparatus that is at atmosphericpressure. The rolling element bearing 50 and second portion 26 of theshaft 20 are preferably located on the atmospheric pressure side of themagnetic fluid seal. The magnetic fluid seal may also be used toseparate a region of corrosive or toxic gas from a region exposed to theatmosphere or environment.

The rolling element bearing 50 is of the “negative clearance” type inorder to eliminate shaft 20 wobble within the magnetic structure 35 whenthe magnetic fluid sealing device 10 is fully assembled. Conventionalmanufacturing tolerances for the bearing's inner rings and shafts leadto some combinations of an inner ring and shaft in which the inner ringhas been radially strained, thereby, increasing the effective negativeclearance in the bearing. When the shaft and bearing are mounted in thehousing, a large negative clearance can lead to large bearing loadingand high torque being required to turn the bearing. One method ofavoiding this situation is to select shafts and bearings as a matchedset, in which such radial straining has not occurred. Although such aselection process could be effective, it will also be very expensive.

According to one aspect of the present disclosure, an improved method toavoid this situation is to control the shaft manufacturing such that theshafts 20 with the largest diameters are suitably matched to bearings 50with inner rings 55 that have the smallest inner diameter. However, inthis method, all of the other shaft/bearing sets will have a looser fit(e.g., larger inner diameter of the inner ring 55 and smaller diameterof the shaft 20). The magnetic fluid sealing device 10 of the presentdisclosure provides a means to easily overcome this situation.

Referring again to FIGS. 1 and 2A, the second portion 26 or bearingjournal in the shaft 20 is made with at least one groove 30 into which acompressible ring 70 is seated. When inserted into the bearing 50, thecompressible ring 70 is squeezed out to partially fill the spaceavailable in the second radial gap 65 established between the innersurface 62 of the bearing's inner ring 55 and the outer surface 22 ofthe shaft 20. This provides approximate alignment of the shaft 20 withthe bearing 50 along its longitudinal axis. The amount of radial forcerequired to compress the ring 70 is not enough to radially strain thebearing's inner ring 55.

One skilled-in-the-art will understand that the compressible ring 70 maybe made from any known elastomeric or polymeric resin system, includingbut not limited to one selected from the group of epoxies, polyesters,butyl rubbers, silicones, polyethers, polyurethanes, polyolefins,styrene block copolymers, polyvinyl chlorides, and mixtures orcopolymers thereof. The compressible ring 70 is substantially similar toan o-ring. The compressible ring 70, which is seated in the groove 30present in the second portion 26 of the shaft 20, has a cross-sectionaldiameter that is larger than the depth of the groove 30.

To set the final alignment, a liquid locking material 75 is added tofill any remaining small openings in the second radial gap 65established between the outer surface 22 of the shaft 20 and innersurface 62 of the bearing's inner ring 55 as shown in FIGS. 1 and 2A.This final alignment may be accomplished if desired by mounting theshaft/bearing assembly in a fixture that forces the shaft 20 and thebearing 50 into the desired final relationship. When the lockingmaterial 75 has hardened the shaft/bearing assembly is removed from thefixture and can be used in a magnetic liquid sealing device 10.

The hardened locking material 75 couples the shaft 20 and thecompressible ring 70 to the inner surface 62 of the inner ring 55 of therolling element bearing 50. Once the shaft 20 and rolling elementbearing 50 are coupled together by the hardened locking material 75, thestiffness of this coupling is capable of providing the resistancenecessary to offset the tilting of the shaft 20 upon the occurrence of aradial loading or force moment. Tilting of the shaft 20 should beavoided because such an occurrence would move the shaft 20 substantiallyoff-center with respect to the magnetic structure 35, thereby, weakeningthe dynamic seal.

The liquid locking material 75 may be any hardenable or curableadhesive, sealant, or other material system known to oneskilled-in-the-art. The liquid locking material 75 is selected to becompatible with the elastomeric or polymeric resin system that comprisesthe compressible ring 70. Several examples of adhesive and sealantsystems that may be used according to the teachings of the presentdisclosure include, but are not limited to epoxy, polyester, silicone,polyether, polyurethane, and acrylic adhesives and sealants, as well asmixtures or copolymers thereof.

The compressible ring 70 of the present invention is very useful inperforming the centering function. This ring 70 centers the shaft 20along its longitudinal axis (x) with the inner ring 55 of the rollingelement bearing 50. The centering of a magnetic sealing device 10 abouta rotatable shaft 20 is affected by the accuracy of the registrationthat one establishes between the shaft 20 and support bearing 50, aswell as between the shaft 20 and the magnetic structure 35. Since themagnetic structure 35 and the support bearing 50 are usually coupled ina common housing 15, the accuracy of centering the rotatable shaft 20relies upon the accuracy of centering the shaft 20 in the housing 15.

The centering of the shaft 20 in the housing 15 is accomplished byensuring that the center of the concentric circle established by theinner surface 62 of the inner ring 55 and the center of the concentriccircle established by the magnetic structure 35 are coaxially alignedupon their being coupled with the housing 15. The alignment of the shaft20 by the compressible ring 70 also allows for the formation of a small,substantially circumferentially uniform first radial gap 40 between theinner surface of the magnetic structure 35 and the outer surface 22 of ashaft 20.

Referring now to FIG. 2B, the shaft 20 in its second portion 26 mayoptionally comprise a shoulder 80 that encircles the shaft 20 and isinherently formed therewith. Preferably, the surface 82 of the shoulder80 is square or normal to longitudinal axis (x) of the shaft 20 andtherefore can be used to assist in the alignment of the shaft 20 and theinner ring 55 of roller bearing 50. In this case, the use of a fixtureto maintain alignment while the locking material hardens is notnecessary because the shoulder 80 can be used to align the shaft 20 andthe bearing 50, while the compressible ring 70 holds the shaft 20 andbearing 50 together during the time required for the locking material 75to harden. However, a fixture can still be utilized when desirable.

The shaft 20 may be substantially flat along its longitudinal axis (x)before, after, and throughout the region in which the second radial gap65 is established with the inner ring 55 of the roller bearing 50 asshown in FIG. 2A. Optionally the shaft 20 may further comprise a raisedportion 78 that establishes the second radial gap 65 with the inner ring55 of the roller bearing 50 as shown in FIG. 2B. With respect to thislatter case, the groove 30 and compressible ring 70 are located withinthe raised portion 78 of the shaft 20.

Referring now to FIG. 2C, the magnetic sealing device 10 according toanother aspect of the present disclosure may include the ability toself-align the shaft 20 and the rolling element bearing 50 without theuse of a compressible ring. This self-alignment mechanism involves onlythe shaft 20 having a shoulder 80 that is square or normal to theshaft's longitudinal axis (x). The bearing 50 is mounted solidly againstthis shoulder, ensuring that the shaft 20 and bearing 50 axes areparallel. The shaft 20 may optionally comprise a raised portion 78 aspreviously described.

More specifically, the magnetic fluid sealing device 10 comprises ahousing 15 mounted in the apparatus; a shaft 20 rotatably mounted in thehousing 15; a magnetic structure 35; a magnetic fluid 45; and a rollingelement bearing 50. The shaft 20 has an outer surface 22 that defines afirst portion 24 and a second portion 26 along its longitudinal axis(x). Referring to FIG. 2C, the outer surface 22 of the shaft 20 in thesecond portion 26 has a shoulder 80 that encircles the shaft 20. Thesurface 82 of the shoulder 80 is square or normal to the longitudinalaxis (x) of the shaft 20.

The rolling element bearing 50 has an inner ring 55 and an outer ringwith the outer ring 60 being coupled to the housing 15 and the innerring 55 being rotatable in relation to the outer ring 60. The inner ring55 has an inner surface 62 that is sized to encircle the outer surface22 of the shaft 20 along its second portion 26 and to contact the outersurface 22 of shaft 20. The contact that occurs between the innersurface 62 of the inner ring 55 and the outer surface 22 of shaft 20radially aligns and centers the shaft 20 along its longitudinal axis (x)with the rolling element bearing 50.

In this case, the absence of a compressible ring that can hold the shaft20 and bearing 50 together while the locking material 75 hardens can beovercome through the use of an external fixture. Such a fixture willneed to hold the shaft 20 and bearing 50 in accurate alignment while thelocking material 75 hardens. Although possible, the use of an accuratefixture may not be desirable in many applications due to the costassociated with the fixture and the difficulty associated with utilizingthe fixture in a full-scale, mass production environment.

One skilled-in-the-art will understand that the centering mechanism ofthe present invention may be utilized in a variety of applications inwhich a magnetic fluid sealing device 10 provides a ferrofluid sealaround a rotatable shaft 20. Examples of such applications include, butare not limited to, sputtering systems, CVD equipment, ion implantationequipment, etching systems, x-ray apparatus, epitaxial growth systems,and vacuum transport systems.

During the operation of the apparatus in which the shaft 20 is rotatedand the magnetic sealing device 10 provides a sealing function, thegeneration of heat will occur. In order for the magnetic seal tofunction as desired, preferably, the generation of heat is eitherminimized or removed from the device 10. Thus the magnetic sealingdevice 10 may further comprise a combination of thermal resistors,shunts, and heat sinks to reduce or minimize any build-up of heat withinthe device 10. In addition, the housing 15 of the magnetic sealingdevice 10 may include a water cooling mechanism (not shown) that willremove heat generated in the device 10 when the shaft 20 is rotated.

It is another objective of the present disclosure to provide a method ofassembling a magnetic sealing device 10 for use in an apparatus havingrotatable components. Referring to FIG. 3, this method 100 comprises thestep of providing 105 a shaft 20 having an outer surface 22 that definesa first portion 24 and a second portion 26 along its longitudinal axis(x). With respect to the first portion 24, the method 100 furthercomprises the step of providing 120 a magnetic structure 35 having aninner diameter 37 that is sized to encircle the outer surface 22 of theshaft 20, thereby creating a first radial gap 40.

With respect to the second portion 26, the method 100 comprises the stepof providing 110 at least one groove 30 that encircles the shaft 20 andplaces 115 a compressible ring 70 in the groove 30. The cross-sectionaldiameter of the compressible ring 70 is greater than the depth of thegroove 30. The method 100 further includes the step of providing 125 arolling element bearing 50 having an inner ring 55 and an outer ring 60.The inner ring 55 has an inner diameter 62 sized to encircle the outersurface 22 of the shaft 20, thereby, creating a second radial gap 65.

The method 100 further includes the step of providing 130 a housing 15to which the outer diameter 38 of the magnetic structure 35 and theouter ring 60 of the rolling element bearing 50 is coupled. The step ofcoupling 135 the magnetic structure 35 and roller bearing 50 to thehousing 15 is done such that the radial center of the annular magneticstructure 35 is coaxially aligned with the radial center of surface theannular inner ring 55 of the rolling element bearing 50.

The method 100 further comprises the step of inserting 140 the shaft 20through the inner ring 55 of the rolling element bearing 50 such thatthe compressible ring 70 fills part of the second radial gap 65 andmakes contact with the inner ring 55 of the rolling element bearing 50.Thus the compressible ring 70 radially aligns and centers the shaft 20along its longitudinal axis (x) with the rolling element bearing 50.

The method 100 also includes the step of inserting 145 the shaft 20through the inner surface 37 of the magnetic structure 35. The method100 further comprises the steps of adding 150 liquid locking material 75into the second gap 65 and allowing 155 the locking material 75 toharden. The hardened locking material 75 couples the shaft 20 andcompressible ring 70 to the inner ring 55 of the rolling element bearing50. Finally, the method 100 further includes placing 160 a magneticfluid 45 into the first gap 40, wherein the magnetic fluid 45establishes a seal between the magnetic structure 35 and the shaft 30.

According to another aspect of the present disclosure, the method 100may optionally include the step of placing 165 the magnetic fluidsealing device 10 into an external fixture in order to hold the shaft 20and rolling element bearing 50 in radial alignment while the lockingmaterial 75 hardens. Once the locking material 75 hardens, the method100 further includes the step of removing 170 the magnetic fluid sealingdevice 10 from the fixture.

According to yet another aspect of the present disclosure, the method100 may further comprise the step of cooling 175 the housing 15 toremove heat generated in the device 10 when the shaft 20 is rotated.

Another embodiment of the present disclosure provides a method 200 forassembling a magnetic fluid sealing device 10 in which theself-assembling mechanism of FIG. 2C where only a shoulder 80 on theshaft 20 is used to align the shaft 20 and roller bearing 50. Referringto FIG. 4, this method 200 generally includes the step of providing 205a shaft 20 having an outer surface 22 that defines a first portion 24and a second portion 26 along its longitudinal axis (x). A shoulder 80is provided on the outer surface 22 of the shaft 20 in the secondportion 26. The surface 82 of the shoulder 80 is square or normal to thelongitudinal axis (x) of the shaft 20.

The method 200 further comprises the step of providing 220 a magneticstructure 35 having an inner surface 37. The inner surface 37 of thestructure 35 is sized to encircle the outer surface 22 of the shaft 20along its first portion 24, thereby creating a first radial gap 40.

The method 200 further includes the step of providing 225 a rollingelement bearing 50 having an inner ring 55 and an outer ring 60; theinner ring 55 having an inner surface 62 sized to encircle the outersurface 22 of the shaft 20 along its second portion 26.

The method 200 further includes the step of providing 230 a housing 15to which the magnetic structure 35 and the outer ring 55 of the rollingelement bearing 50 is coupled. The step of coupling 235 the magneticstructure 35 and roller bearing 50 to the housing 15 is done such thatthe radial center for the annular magnetic structure 35 is coaxiallyaligned with the radial center for the annular inner ring 55 of therolling element bearing 50.

The method 200 also comprises the step of inserting 240 the shaft 20through the inner ring 55 of the rolling element bearing 50 such thatsurface 82 of the shoulder 80 on the shaft 20 makes contact with theinner surface 62 of the inner ring 55 of the rolling element bearing 50.The contact between the shoulder 80 on the shaft 20 and the inner ring55 of the rolling element bearing 50 axially aligns the shaft 20 alongits longitudinal axis (x) with the rolling element bearing 50.

The method 200 further includes inserting 245 the shaft 20 through theinner surface 37 of the magnetic structure 35. The method 200 furthercomprises the steps of adding 250 liquid locking material 75 into thesecond gap 65 and allowing 255 the locking material 75 to harden. Themethod 200 also includes the step of placing 265 the magnetic fluidsealing device 10 into an external fixture in order to hold the shaft 20and rolling element bearing 50 in radial alignment while the lockingmaterial 75 hardens. The hardened locking material 75 couples the shaft20 to the inner ring 55 of the rolling element bearing 50. Once thelocking material 75 hardens, the method 200 further includes the stepsof removing 270 the magnetic fluid sealing device 10 from the fixtureand placing 160 a magnetic fluid 45 into the first radial gap 40. Themagnetic fluid 45 establishes a seal between the magnetic structure 35and the shaft 20.

Optionally, the method 200 may further comprise the step of cooling 275the housing 15 to remove heat generated in the device 10 when the shaft20 is rotated.

A person skilled in the art will recognize that any measurementsdescribed are standard measurements that can be obtained by a variety ofdifferent test methods. Any test methods described in the presentdisclosure represent only one available method to obtain each of thedesired measurements.

The foregoing description of various embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the preciseembodiments disclosed. Numerous modifications or variations are possiblein light of the above teachings. The embodiments discussed were chosenand described to provide the best illustration of the principles of theinvention and its practical application to thereby enable one ofordinary skill in the art to utilize the invention in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

1. A magnetic fluid sealing device for incorporation into an apparatushaving rotatable components; the sealing device comprises: a housingmounted to the apparatus; a shaft rotatably mounted in the housing; theshaft having an outer surface that defines a first portion and a secondportion along its longitudinal axis (x); the outer surface of the shaftin the second portion having at least one groove that encircles theshaft; a magnetic structure having an inner surface; the structure beingcoupled to the housing; the inner diameter of the structure sized toencircle the outer surface of the shaft along its first portion, therebycreating a first radial gap; a magnetic fluid located in the first gap;one rolling element bearing having an inner ring and an outer ring; theouter ring being coupled to the housing; the inner ring having an innerdiameter sized to encircle the outer surface of the shaft along itssecond portion, thereby creating a second radial gap; and a compressiblering located in the groove of the shaft; the compressible ring adaptedto fill a part of the second gap and contact the inner ring of therolling element bearing; and a hardenable locking material filling apart of the second gap, the hardenable locking material extendingaxially beyond the groove; wherein the compressible ring radially alignsand centers the shaft along its longitudinal axis (x) with the rollingelement bearing; wherein when the locking material hardens, it couplesthe shaft and compressible ring to the inner ring of the rolling elementbearing; wherein the magnetic fluid establishes a seal between themagnetic structure and the shaft.
 2. The magnetic fluid sealing deviceof claim 1, wherein the shaft in the second portion further comprises ashoulder that encircles the shaft and is intimately formed therewith;the surface of the shoulder being normal to the longitudinal axis (x) ofthe shaft.
 3. The magnetic fluid sealing device of claim 1, wherein themagnetic structure further comprises a combination of pole pieces andmagnets with the pole pieces and the outer surface of the shaft in itsfirst portion being made from a magnetically permeable material; whereinthe magnets remain stationary when the shaft is rotated.
 4. The magneticfluid sealing device of claim 1, wherein the magnetic structure furthercomprises a single pole piece or a combination of pole pieces made froma magnetically permeable material and the first portion of the shaftincludes a single pole piece or combination of pole pieces and magnetswith the pole piece being made of a magnetically permeable material;wherein the magnets rotate with the shaft.
 5. The magnetic fluid sealingdevice of claim 1, wherein the shaft has a hollow core defining apassageway, channel, duct, or conduit.
 6. The magnetic fluid sealingdevice of claim 1, wherein the seal established between the magneticstructure and the shaft separates a low pressure region in the apparatusfrom a region in the apparatus that is at atmospheric pressure.
 7. Themagnetic fluid sealing device of claim 6, wherein the region atatmospheric pressure includes the rolling element bearing and the secondportion of the shaft.
 8. The magnetic fluid sealing device of claim 1,wherein the compressible-ring is made from a polymer selected as onefrom the group of epoxies, polyesters, butyl rubbers, silicones,polyethers, polyurethanes, polyolefins, styrene block copolymers,polyvinyl chlorides, and mixtures or copolymers thereof.
 9. The magneticfluid sealing device of claim 1, wherein the cross-sectional diameter ofthe compressible-ring is greater than the depth of the groove in theshaft.
 10. The magnetic fluid sealing device of claim 1, wherein theinner ring of the rolling element bearing is rotatable in relation tothe outer ring.
 11. The magnetic fluid sealing device of claim 1,wherein the housing further comprises a water cooling mechanism toremove the heat generated in the device when the shaft is rotated.
 12. Amethod for assembling a magnetic fluid sealing device for use in anapparatus having rotatable components, the method comprising the stepsof: providing a shaft rotatably mounted in the housing having an outersurface that defines a first portion and a second portion along itslongitudinal axis (x); providing at least one groove that encircles theshaft in the second portion of the shaft; placing a compressible ring inthe groove of the shaft, the cross-sectional diameter of thecompressible ring being greater than the depth of the groove; providinga magnetic structure having an inner surface; the inner diameter of thestructure is sized to encircle the outer surface of the shaft along itsfirst portion, thereby creating a first radial gap; providing a rollingelement bearing having an inner ring and an outer ring; the inner ringhaving an inner diameter sized to encircle the outer surface of theshaft along its second portion, thereby creating a second radial gap;providing a housing mounted to the apparatus; coupling the magneticstructure and the outer ring of the rolling element bearing to thehousing such that the radial center of the annular magnetic structure iscoaxially aligned with the radial center of the annular rolling elementbearing; inserting the shaft through the inner ring of the rollingelement bearing such that the compressible ring fills part of the secondgap and makes contact with the inner ring; inserting the shaft throughthe inner diameter of the magnetic structure; adding liquid lockingmaterial into the second gap such that the locking material extendsaxially beyond the groove; allowing the locking material to harden;placing a magnetic fluid into the first gap; wherein the compressiblering radially aligns and centers the shaft along its longitudinal axis(x) with the rolling element bearing; wherein the hardened lockingmaterial couples the shaft and compressible ring to the inner ring ofthe rolling element bearing; wherein the magnetic fluid establishes aseal between the magnetic structure and the shaft.
 13. The method ofclaim 12, wherein the method further comprises the steps of: placing themagnetic fluid sealing device in a fixture to hold the shaft and rollingelement bearing in radial alignment while the locking material hardens;and removing the magnetic fluid sealing device from the fixture afterthe locking material has hardened.
 14. The method of claim 12, whereinthe step of providing a shaft having an outer surface that defines afirst portion and a second portion along its longitudinal axis (x)further comprises a shoulder that encircles the shaft and is intimatelyformed therewith; the surface of the shoulder being normal to thelongitudinal axis (x) of the shaft; wherein the shoulder in combinationwith the compressible ring holds the shaft in radial and axial alignmentwith the rolling element bearing while the locking material hardens. 15.The method of claim 12, wherein the method further comprises the stepof: cooling the housing to remove heat generated in the device when theshaft is rotated.